An intense scientific debate has opened up over whether genetically modified (GM) crops in Mexico have contaminated wild maize (corn).

Last November, the magazine Nature published data from two authors who said they had detected DNA from GM plants in wild crops growing in a remote area.

But the editor says complaints about what were perceived to be flaws in the authors' experimental procedures have now led him to conclude that the paper should not have been published.

Despite that, the authors stand by their report, and have published new information to support it.

The issue is an important one because it goes right to the heart of the debate on how modern crops will impact on the environment.

Threatened diversity

The scientists at the centre of it all are Ignacio Chapela and David Quist of the University of California, Berkeley, US.

They compared wild maize from the Sierra Norte de Oaxaca mountains in Mexico with GM varieties from the Monsanto company in the US and with samples known to be uncontaminated.

Protests against GM crops remain strong

They were puzzled by their finding that some of the wild samples - which were growing around 100 kilometres (62 miles) from the nearest industrially farmed crops - were contaminated with telltale sections of DNA from GM crops.

They concluded that food aid, perhaps from the US, was probably responsible.

Mexico has had a moratorium on new plantings of GM maize since 1998 but allows the import of GM crops for consumption.

Chapela and Quist expressed the concern that the invasion of GM could threaten the valuable diversity of native wild maize.

Left to readers

In the current issue of Nature, though, the editor says he received "several criticisms" of the paper.

He writes: "The authors have now obtained some additional data, but there is disagreement between them and a referee as to whether these results significantly bolster their argument.

"Nature has concluded that the evidence available is not sufficient to justify the publication of the original paper.

"As the authors nevertheless wish to stand by the available evidence for their conclusions, we feel it best simply to make these circumstances clear, to publish the criticisms, the authors' response and new data, and to allow our readers to judge the science for themselves."

The criticisms of Quist and Chapela concentrate on their use of a method known as i-PCR, inverse polymerase chain reaction - a technique used to amplify samples to sufficient levels so that they can be more easily studied.

'Unwarranted conclusion'

Matthew Metz, of the University of Washington, Seattle, US, and Johannes Futterer, of the Institute of Plant Sciences, Zurich, Switzerland, say the evidence adduced by Quist and Chapela for GM contamination "is based on the artefactual results of a flawed assay..."

They write: "Transgenic corn may or may not be hybridising to traditional maize cultivars in Mexico.

GM protests know no borders

"Whether these events will result in introgression of traits, and whether such introgression could have a negative effect on crop diversity, is pure speculation..."

Likewise, another group of critics, led by Nick Kaplinsky of the University of California, Berkeley, says the conclusion of the original paper "seems to be based on an artefact arising from the i-PCR [Quist and Chapela] used..."

This group concludes: "Transgenic corn may be being grown illegally in Mexico, but Quist and Chapela's claim that these transgenes have pervaded the entire native maize genome is unfounded.

Unyielding defence

"It is important for information about GM organisms to be reliable and accurate, as important policy decisions are at stake."

In their response, Quist and Chapela acknowledge that "PCR-based methods are sensitive and therefore open to artefacts".

But they say they "strongly disagree that the presence of these artefacts is unavoidable or uncontrollable".

"The consistent performance of our controls, as reported, discounts beyond reasonable doubt the possibility of false positives in our results."

They conclude that their most recent work "confirms our original detection of transgenic DNA integrated into the genomes of local landraces in Oaxaca."